The present invention relates in general to communication systems and components therefor, and is particularly directed to a reduced complexity method and apparatus for controlling the DC bias and modulation signal drive level to an external laser modulator, such as an X-cut lithium niobate (LN) Mach Zehnder (MZ) modulator, using phase quadrature components of a single reference frequency.
A typical external laser modulator architecture for a digital optical transmitter, such as that employed for use in a very high data rate (e.g., on the order of 10 Gb/s or higher) communication system, is diagrammatically illustrated in FIG. 1 as comprising a laser modulator 10, such as an X-cut lithium niobate (LN) Mach Zehnder (MZ) modulator, that is external to and disposed in the output beam path 12 of a continuous wave laser 14. The LN MZ modulator 10 has a first, drive signal port 11 to which a digital drive signal is applied, and a second, DC bias port 13, to which a DC bias voltage is coupled. The drive signal is supplied from an output 23 of an analog driver 20, which has a signal input 21 coupled to a digital RF signal source 22 and a drive level (gain) control port 24 coupled to receive a drive amplitude control voltage. The DC bias voltage is supplied from an output 33 of a bias controller 30, which as a DC voltage input 31 coupled to receive a DC control voltage.
In an effort to sustain long-term, stable operation of the laser modulator 10, compensating for environmental effects, such as temperature and agingxe2x80x94which is especially important for telecom and datacom applications, the DC control voltage to the DC bias controller 30 is coupled to a first reference oscillator tone-based closed loop control path 35, that monitors the beam output of the laser modulator, and adjusts the control voltage input to the bias controller 30, as necessary, to ensure that the DC voltage bias necessary for proper modulator operation is coupled to DC bias port. In addition, in order for the modulator drive signal to track changes in the modulation efficiency slope, a second, reference oscillator tone-based closed loop control path 25 is coupled to monitor the output of the laser modulator. This second closed loop control path is operative to controllably adjust the DC input to the analog driver 20, in order to maintain a constant optical extinction ratio of the modulator.
Because this conventional architecture employs two different tones/frequencies for signal drive control and DC bias adjustment, not only are two different control channel circuits required, but they must include filter/isolation circuitry that successfully reduces/minimizes interaction or cross-coupling of one control channel into the other. For an illustration of non-limiting examples of patent literature describing prior art laser modulator architectures, including a dual tone mechanism of the type employed in FIG. 1, attention may be directed to the following U.S. patents: U.S. Pat. Nos. 5,317,443; 5,742,268; 5,805,328; 5,917,637; 5,907,426; 5,400,417; 5,003,264; 5,343,324; 5,453,608; 5,900,621; 5,440,113; 5,170,274; 5,208,817; and 5,726,794.
In accordance with the present invention, shortcomings of conventional laser modulator control stabilization schemes, including those employed in the systems of the above-referenced patent literature, are effectively obviated by controlling the DC bias and modulation signal drive level to an external LN MZ laser modulator, using relatively orthogonal laser output monitoring feedback control loops associated with phase quadrature components of a single reference frequency tone. A signal driver to which a digital RF signal is applied is coupled to receive a drive amplitude control voltage from the output of an associated phase detector of a Q-channel synchronous detector feedback loop. In addition, a DC bias controller is coupled to receive a DC control voltage from the output of an associated phase detector of an I-channel synchronous detector feedback loop.
Each feedback loop is coupled through a loop filter of a common feedback path is electro-optically coupled to monitor the optical output of the laser modulator. The Q and I channel synchronous detector feedback loops are referenced to phase-quadrature tone components of a single frequency/tone generator. The quadrature-phase and in-phase frequency components are respectively coupled to the respective Q and I channel phase detectors, which are also coupled to the outputs of synchronous demodulators for the Q and I channel feedback loops. The phase detectors produce respective error voltages that close Q and I channel feedback loops and control the optical extinction ratio and DC bias voltage of the modulator.
The Q channel and I channel synchronous demodulators are coupled to the output of a loop filter of an opto-electronic detector path, which monitors the modulated optical output beam from the laser modulator. A monitored photo-current is coupled to a transimpedance amplifier, which provides an output voltage signal representative of the monitored optical signal, and couples that signal through a loop filter to the Q and I channel synchronous demodulators. The outputs of the Q and I channel synchronous demodulators are compared in their associated phase detectors with their respective relative quadrature reference tones sourced from the tone generator. The phase detectors produce respective DC control voltages to the modulation signal driver and the DC bias controller for controllably adjusting the operation of the laser modulator, so as to drive the respective phase differences of the quadrature tone signal components applied to the two phase detectors to or very close to a null condition. Namely, both a constant optical extinction ratio and DC biasing of the laser modulator to compensate for variations in environmental conditions are simultaneously realized by means of a single reference tone generator, which reduces the circuit complexity necessary to minimize mutual interaction of the two control channels.